Ultimately, wastewater, or spent water, must be returned to the land or the waters. Considerable engineering research and development has focused on the complex question of which contaminants occur in wastewater and the extent to which they must be removed to protect the environment.
In recent years, much of this research and development has been directed to the treatment of wastewater resulting from processes employed in industrial plants. The variety and amount of industrial wastes discharged into the environment and into municipal sewage systems has increased significantly during the past few decades. The concentration of contaminants in these wastes is often very high, and regulations for environmental protection now require that industrial wastes be treated at their point of generation to reduce their contaminant concentration to an acceptably low level before allowing their discharge into municipal treatment systems or the environment. Management of such wastes requires analysis of the particular local conditions, the degree of contaminant removal (treatment) required before the wastewater can be reused or discharged to a larger (municipal) system or discharged to the environment, and the operations and processes necessary to achieve that required degree of treatment.
The characterization of a particular wastewater depends on its physical, chemical and biological constituents. The most important physical characteristic of wastewater is its total solids content, which is the total amount of matter in suspension, colloidal matter, and matter in solution. The total solids content is defined as all the matter that remains as residue upon evaporation of the wastewater to dryness at 103.degree. to 105.degree. C.
The total solids can be subclassified as suspended solids and filterable solids. Suspended solids include settleable solids and all floating materials, whether floating on the surface or in the body of the wastewater, while filterable solids include the colloidal and dissolved solids. The colloidal solids cannot be removed by settling, but usually require biological oxidation or coagulation followed by sedimentation for removal from the wastewater. The dissolved solids consist of both organic and inorganic molecules and ions that are present in true solution in the water. Other physical characteristics include temperature, color and odor.
The chemical constituents of wastewater are typically divided into three categories, organic matter, inorganic matter and dissolved gases. Organic matter generally includes proteins, carbohydrates, fats, oils, and greases, surfactants, pesticides, herbicides and other agricultural chemicals and substances. Inorganic matter usually includes minerals and inorganic ions, such as phosphates. Gases commonly found in wastewaters include nitrogen, oxygen, carbon dioxide, hydrogen sulfide, ammonia and methane. The latter three gases are usually derived from the decomposition of the organic matter present in the wastewater.
The biological constituents of wastewater include the microorganisms found in the wastewater. These microorganisms include protista, such as bacteria and protozoa. As explained hereinafter, biological treatment of wastewater depends on the establishment and maintenance in a treatment system of a population of naturally occurring microorganisms adequate to perform oxidation of the organic matter.
The objectionable properties of wastewater derive, in large part, from the organic matter. Organic matter may be "stable" or "unstable". Stable compounds are fairly resistant to bacterial breakdown, while unstable are not. The objective of treatment of the organic matter is to stabilize the organic matter by oxidation, and a measure of the amount of oxygen required to accomplish this action gives a measure of the amount of organic matter contaminating the wastewater.
The most common measure of organic pollution of wastewater is the biochemical oxygen demand (BOD). The BOD is the quantity of dissolved oxygen required by microorganisms for biochemical oxidation of the organic matter in a given time at a given temperature. The efficiency of a treatment system is usually evaluated on the basis of BOD removal by the system. The BOD is usually given as BOD.sub.5, the biochemical oxygen demand for five days at 20.degree. C. The BOD is typically given in milligrams/liter (mg/l) or parts per million (ppm). For dilute systems, in which a liter of wastewater has a mass nearly equal to a kilogram, the mg/l unit is interchangeable with ppm.
A BOD.sub.5 of about 200-400 mg/l is typically required in a wastewater for it to be acceptable for discharge into a sewage system, such as a municipal (i.e., a community, public or governmental) system, where additional treatment will further reduce BOD.sub.5, e.g., typically to levels acceptable for discharge into streams, lakes, or other bodies of water in the environment. A BOD.sub.5 of less than about 5-10 mg/l is typically required for treated wastewater to be acceptable for discharge directly into the environment (e.g., a stream or lake).
Another wastewater contaminant, which is of considerable concern with respect to quality of water discharged to the environment, is phosphates. A phosphate concentration of less than about 1 mg/ml (i.e., 1 ppm) as PO.sub.4.sup.-3 is desirable, and may be required, for treated wastewater to be acceptable for discharge directly into streams, rivers, lakes, or the like, in the environment.
Contaminants in wastewater are removed by physical, chemical and biological methods. Physical methods include sedimentation (settling), filtration and floatation. Sedimentation is the simplest and most widely used physical treatment method. Much of the organic matter in wastewater is in a suspended form rather than in solution and removal of the sediment or sludge brings about a large reduction in BOD of the wastewater.
Filtration serves the same purpose as sedimentation. Suspended and colloidal solids in wastewater may be removed by filtration, a process which allows the water to pass through a bed of, e.g., sand, or a combination of granular materials. The removal of substances appears to be a combination of physical and chemical processes, such as straining phenomena and adsorption. Filtration is faster than sedimentation, requires less space, and the retained solids contain less water.
Floatation is used to concentrate oils, grease, and fine dispersed solids on the surface of the wastewater. For oils and grease, a grease trap or skimming tank is common. Such a device is simply a detention tank which reduces the flow velocity of the wastewater and allows time for oil and grease globules to rise to the surface and collect as an oil layer during the period that the wastewater, with reduced flow rate, is detained in the tank. Removal can be effected by scraping techniques. Dissolved air floatation (DAF) methods are also used to promote flocculation and remove a surface or float sludge. Minute bubbles of air are used to bring finely dispersed solids, including microorganisms, to the surface of the wastewater where they are removed by skimming or scraping techniques. The flocculation may be aided by a flocculating or coagulating agent, as is explained hereinafter.
Chemical methods include neutralization and aeration/oxidation. An important factor in the treatment of all aqueous wastes is to produce a final effluent with a neutral pH of approximately 7. Many industrial operations produce acidic (low pH) or alkaline (high pH) wastewaters. Various substances must be added to bring the pH into the neutral range.
Aeration is a process used in nearly all types of wastewater treatment. Aeration is used to provide dissolved oxygen for biological oxidation of the organic matter by microorganisms, but aeration also facilitates release of certain volatile substances to the atmosphere and is effective in removing certain other organic compounds by oxidation, such as phenols, sulfides and sulfites. Aeration may also be used to make a float sludge by adding buoyancy to sludge particles with bubbles of air.
Biological methods of waste treatment constitute the most common and widely used methods because they are the most economical means of accomplishing an acceptable final effluent. They utilize naturally occurring microorganisms to accomplish results which would be quite costly if attempted by chemical or mechanical means. The microorganisms are used to bring about a breakdown of complex organic compounds primarily by oxidation (and hydrolysis). Complete aerobic decomposition results in compounds which, under ordinary conditions of temperature and pressure, are stable, e.g., water, carbon dioxide, nitrogen, chlorides, nitrates, etc.
One aerobic process is the use of a trickling filter. The trickling filter consists of a bed of highly permeable media to which microorganisms are attached and through which the wastewater is percolated. The microorganisms covering the surface of the filter media use the wastewater as a food source. The surface area of the media supporting the growth of organisms is the effective part of the system. The biological growth and activity depend on a constant supply of dissolved oxygen. The effluent from the filter carries with it living and dead organisms and waste products of the biological reactions. These sludge flocs are indicators of the efficient functioning of the trickling filter and are separated from the water in settling tanks.
Another aerobic process is the activated sludge method which is the most widely used method to bring about stabilization in wastewater having organic matter constituents. The method depends on establishing and maintaining a population of degrading microorganisms and providing close contact of the degrading microorganisms and a supply of dissolved oxygen. The microorganisms feed and grow upon the oxidizable material in the wastewater and form a suspended floc of "activated sludge" in the water. Air bubbled through the water or absorbed by constantly renewing the air-water interface (by agitation) replenishes the oxygen needed for the biological oxidation. The mixture of wastewater and activated sludge, known as "mixed liquor", is then settled to separate the activated sludge solids from the treated (i.e., reduced BOD) water. The settled activated sludge is usually mechanically returned (by pump) to the aeration site (usually a tank or vessel).
The solids in an activated sludge system tend to build up due to accumulation of inert material and the growth of the microorganisms. To control the amount of solids during aeration, the excess solids, i.e., "excess sludge", are wasted from the system regularly. Typically, the influent wastewater is mixed with about 20 to 30 percent by volume of activated sludge and approximately the same number of pounds of suspended solids which enter the treatment system each day must be wasted as excess activated sludge.
Disposal of the excess activated sludge usually requires additional treatment of the sludge because the BOD of the sludge may run higher than 3000 mg/l. Generally, the excess sludge is "digested" aerobically or anaerobically, i.e., conversion of the organic matter in the sludge to more stable compounds. Digestion operates when available food for the microorganisms is at a minimum and the microorganisms are in the endogenous phase, where they are forced to metabolize their own protoplasm as a food source. Digestion is generally followed by drying, lagooning, wasting on farm lands, or trucking to larger waste treatment facilities.
The activated sludge method, while the most widely used treatment method, has several operational problems. In conventional activated sludge facilities, the aerators are of a fixed size and are designed to give an average detention period of 6 to 8 hours for aeration, with a return activated sludge rate equal about 20 to 30 percent of the influent wastewater flow. Aeration tanks utilized for the aeration are usually shallow with small cross-sectional area. Unfortunately, average conditions do not generally exist. The flow of wastewater often fluctuates between high flows and low flows. For example, an industrial plant may have peak loads during the day and minimum loads at night and on weekends. The concentration of contaminants typically also fluctuates; often the highest flow will have the highest concentration of contaminants. The matching of food to microorganism, i.e., the fluctuating biological load to the weight of sludge retained, can be a difficult problem.
Another problem is sludge bulking in which a large volume of light, fluffy sludge forms which does not settle. One type of bulking is due to the presence of filamentous microorganisms, such as filamentous bacteria, for example, Sphaerotilus or Leptothrix bacteria, and their growth in excessive numbers causes the sludge to be less dense. (A low density sludge will float.)
Another problem is rising sludge which can occur from overaeration (i.e., quantity of air is too large or aeration period is too long). In this phenomenon, the sludge is initially dense and settles well, but rises in chunks and floats on the surface of the water. This condition is associated with production of nitrogen gas from nitrates and nitrites in the water.
Many modifications of the simple activated sludge process have been described to control some of the operational problems as well as add flexibility and tolerance to a system. Some modifications have attempted to control fluctuations in the quantity and quality of wastewater influent. For example, flow equalization tanks have been described and used which have a sufficiently large capacity to hold the incoming wastewater and provide more uniform composition of the wastewater, and permit an even flow to the aeration/biological oxidation site, preventing "shocking", i.e., sudden increases in contaminant concentration, which can be very deleterious to the microorganisms. See, for example, U.S. Pat. No. 4,894,162 for use of such a tank; see, also, U.S. Pat. No. 3,886,065 which describes a method of metered discharge of final effluent with aeration and clarification vessels capable of receiving widely fluctuating flow rates.
Other modifications have attempted to vary the aeration conditions. Such modifications include contact stabilization, extended aeration and the Kraus process. In contact stabilization, the mixed liquor enters a contact tank where it is aerated for about 30 to 90 minutes, the sludge is then separated and the return sludge aerated in a sludge aeration tank for 3 to 6 hours before being mixed with the influent wastewater. Extended aeration is a process used for low organic loading and long aeration time. Extended aeration works on the endogenous phase of the microorganism growth curve. In the endogenous phase, the microorganisms are forced to metabolize their own protoplasm. Lysis occurs, by which nutrients remaining in dead cells diffuse out to furnish food for the remaining cells. The Kraus process includes a reaeration of a mix of return sludge, digested sludge and digester supernatant prior to mixing with the influent wastewater and its aeration.
Yet another modification includes the use of deep tank aeration. U.S. Pat. Nos. 3,574,331 and 4,374,027 describe the use of deep tanks in which air (or oxygen) is supplied at the bottom of the tank where the hydrostatic pressure of the fluid in the tank is high. The high hydrostatic pressure is described as facilitating and accelerating oxygen transfer to the microorganisms present in the fluid in the tank, providing more efficient processing than in shallow tanks of equal volume.
Various techniques have been described to improve sludge formation and separation from the water. For example, U.S. Pat. Nos. 4,728,517 and 4,282,256 describe dissolved air floatation techniques in which air is dissolved in the mixed liquor under pressure and then allowed to come out of solution in a vessel at atmospheric pressure to produce a float sludge. U.S Pat. No. 4,728,517 injects compressed air in line along with coagulating and flocculating agents, while U.S. Pat. No. 4,282,256 pressurizes the mixed liquor in line but holds it in a retention tank under pressure before release to the flotation tank. U.S. Pat. No. 4,786,413 describes an activated sludge system in which a support material is added to an aeration tank to facilitate flocculation of microorganisms for subsequent settling of the sludge. U.S. Pat. No. 4,406,790 describes use of an aeration tank in which the surface of the contained fluid is heated, preferably by steam, to facilitate bacterial action.
Several methods for treatment of excess sludge have been described. U.S. Pat. No. 3,047,492 describes an activated sludge system with aerobic sludge digestion and chlorination of the water before final discharge to the environment. U.S. Pat. No. 4,406,795 describes a heat/extraction method for separating the solids from the liquid in sludge. U.S. Pat. No. 3,876,436 describes a sludge treatment consisting of wet air oxidation followed by biological oxidation in the presence of activated carbon. U.S. Pat. No. 4,370,235 describes a sludge treatment in which aerobic digestion is preceded by decomposition of the microorganisms present in the sludge, e.g., by treatment with ozone.
Still other modifications include pretreatment of wastewater with neutralizations and flocculating agents prior to a conventional activated sludge circuit (see, Pat. No. 4,894,162), a rapid settling technique to produce low phosphorous compound content effluent (see, U.S. Pat. No. 3,386,910) and use of various polymers for improving sludge formation and processing (see, U.S. Pat. No. 3,397,139).
As described hereinbefore, some industrial wastewaters result in highly concentrated organic loadings and may have high phosphate concentrations (more than about 50 ppm). Such wastes include those derived from meat-packing plants and food-processing, including milk-processing (e.g., dairies, cheese-making), plants. These wastes are particularly conspicuous as having a very high oxygen requirement (BOD) and being especially susceptible to anaerobic decomposition if sufficient oxygen is not provided. For example, wastewaters with high carbohydrate content are especially unstable and susceptible to anaerobic decomposition.
The biological treatment of such wastewaters to reduce BOD has been problematic. In particular, treatment of dairy and other milk-processing wastewater has historically been difficult. Such wastewater has a high carbohydrate content, which favors the growth of bacteria of species of the Sphaerotilus-Leptothrix group found in the wastewater. As described hereinbefore, these bacteria form a filamentous bulking. This bulking tendency has been regarded as a problem in design and operation of systems to treat such wastes because of the high likelihood of producing a floating sludge. Most treatment methods for milk-processing or food-processing wastewater are directed to controlling this bulking. As such, biological treatment of such wastewaters, because of their inherent qualities, remains difficult and expensive. The art has yet to respond with a simple, cost effective, efficient system for reliable treatment.